General Background
Coherent optical radiation, or laser light, is used extensively in communications, medicine, research, imaging, and in many other areas of technology. In such applications, the laser radiation may be used directly or may be used as an intermediate pump source for purposes of promoting amplification or subsequent laser action. When an application's power requirements are small, on the order of several milliwatts more or less, and beam quality is not an overriding concern, laser diodes have been extensively employed because they are directly modulatable and of convenient size and acceptable beam quality. Where modest power is needed, on the order of a watt or so with superior beam quality, such as a diffraction-limited spot, optical fiber lasers with solid state pump sources have been used. For higher power applications where several watts may be needed, as in certain printing applications, use has been made of laser diode arrays as pump sources coupled to special fiber gain structures. For even higher power requirements, ten watts or more, high power laser diode arrays, whose cavities emit a few modes, may be coupled to such specialty gain fibers. However, care must be taken to assure efficient coupling power if maximum power benefit is to be achieved. Since single-mode cores are small, 10 .mu.m or less, and typical materials limit the size of fiber numerical apertures (NA), it is virtually impossible to efficiently couple multimode laser array energy directly into single-mode gain cores via endfire coupling techniques.
As mentioned above, high-power laser light can be obtained by combining the outputs from the emitting cavities of laser diode arrays. However, combining the separate outputs in such a manner that brightness is conserved and power losses are kept to a minimum can be difficult to achieve.
The difficulty results from the fact that the aggregate output of a multiple laser source is not configured for efficient coupling into an optical fiber due to a mismatch between geometric properties and numerical apertures of the source and the receiving fiber. A typical multiple laser source may be a laser bar 10, as shown in FIG. 1. Optical radiation is emitted from a light-emitting face 11 along which are positioned a plurality of laser cavities. In the example provided, light-emitting face 11 includes a laser diode 12 positioned at an array interval spacing 16 from an adjacent laser diode 13. Laser diode 12 typically has an emitting facet output in the shape of a long, narrow rectangle and is oriented with its long dimension parallel to a laser bar axis 14.
For the purpose of illustration, a set of laser-beam uvw-coordinates 19 is used to describe the propagation characteristics of the beams of radiation emitted from the laser diodes. The orientation of the w-axis is perpendicular to light-emitting face 11 and coincident with the direction of propagation of the beams of radiation. Coordinate set 19 "travels" with each beam, rotating about the w-axis as the beam is rotated, and changing direction as the beam's direction of propagation is changed. Laser diode 12 emits a laser beam 20 and laser diode 13 emits a laser beam 21 and so on.
The radiation distributions of the emitted laser beams 20 and 21 are represented by ellipses to indicate that they each have a v-component parallel to laser bar axis 14 and a u-component perpendicular to laser bar axis 14. A more quantitative representation of the laser beam divergence is provided in the graphical illustration of FIG. 2, which shows that each laser beam diverges at a larger angle .theta..sub.u in the u-direction than the angle of divergence in the v-direction .theta..sub.v, as the laser beam propagates in the w-direction. NA values, measured to include 95% of the optical power, are typically 0.30 to 0.70 (17.degree. to 40.degree.) for NA.sub.u and 0.10 to 0.35 (6.degree. to 20.degree.) for NA.sub.v. Before such laser beams can be guided into an optical fiber, a coupling device is needed to reformat the radiation into a more suitable configuration that is more compatible with the geometry and the NA of the fiber.
One such optical coupler is disclosed in U.S. Pat. No. 4,763,975 to Scifres et. al. FIG. 3 shows an optical system 30 which physically combines the outputs of a plurality of laser light sources 32 by means of a plurality of fiberoptic waveguides 34. Each waveguide 34 has an input end 36 flattened so as to more efficiently couple light from the laser light source 32 to the waveguide 34. The waveguides 34 are then stacked at their output ends 38 to collectively emit a less elongated, stacked light beam made up of the individual light beams 40 emitted from the waveguide output ends 38. A lens 44 or other optical means can be used to couple the stacked light beam from waveguide stack 42 into the cavity mode volume of a solid state laser 46.
U.S. Pat. No. 5,268,978 to Po et al. discloses an optical coupling system, similar to optical system 30, in which the fiber optic waveguides are rectangular in cross section throughout their lengths. An alignment block is used to position the waveguides at their input ends and a lens is used to demagnify and image the aggregate outputs at the opposite, stacked ends into the inner cladding of an optical fiber.
It can be appreciated that these methods of combining laser array outputs by means of a fiberoptic waveguide bundle require precise positioning of each waveguide against the output aperture of each laser light source and introduce some additional loss due to absorption in the coupling fibers. A physical misalignment between any of the waveguide ends and the corresponding laser light sources will proportionately decrease the total power delivered to the solid state laser by the waveguide stack.
A measure of coupling mismatch between two optical components can be provided by a quantitative comparison of the "etendu" values for the two components. The etendu of a component is defined as the mathematical product of the angular extent and the spatial extent of the radiation entering or emitting from that component: EQU etendu .DELTA. [angular extent].times.[spatial extent]
To illustrate, assume laser bar 10 to have a linear array of twenty laser diodes on a face 1.00 cm long by 0.1 mm wide. If laser diodes 12 and 13 are one .mu.m in the u-direction and 175 .mu.m in the v-direction, with an array interval spacing 16 of 485 .mu.m center-to-center, NA.sub.u 25 is approximately 0.55 (31.5.degree.) and NA.sub.v 27 is approximately 0.12 (6.9.degree.), as indicated in FIG. 1.
For laser diode 12, the u-component etendu value becomes 1 .mu.m.times.0.55 NA, or 0.55 .mu.m-NA, and the v-component etendu value is 175 .mu.m.times.0.12 NA, or 21 .mu.m-NA. For laser bar 10, the u-component etendu is also 0.55 .mu.m-NA. The v-component etendu for laser bar 10 is 1,200 .mu.m-NA, which is more than two thousand times as great as the u-component etendu. In comparison, the inner cladding of an optical fiber might have an NA of 0.47 and a dimension of 120 .mu.m by 360 .mu.m. This would yield an etendu of 56 .mu.m-NA by 169 .mu.m-NA. Direct coupling of a laser bar, such as laser bar 10, into the optical fiber would not be efficient because the v-component etendu of the laser bar exceeds the largest etendu provided by the optical fiber.
This mismatch cannot be corrected solely by the use of anamorphic imaging systems even though they have different spatial magnification in the two orthogonal directions. Any practical imaging system which decreases the etendu mismatch between a laser diode array and an optical fiber must perform more complicated reformatting tasks such as rotating each emitted diode beam by 90.degree. before optical corrections to the beam are made by the imaging system.
U.S. Pat. No. 5,168,401 to Endriz discloses a prism-and-lens array structure for reimaging the outputs of a multiple laser source. A perspective view of the prism-and-lens array structure is provided in FIG. 13 of the reference patent and presented here in FIG. 4, which has been slightly modified for clarity and for purposes of analysis. As shown, prism-lens device 50 is here depicted as it can be used in conjunction with laser bar 10. An xyz-coordinate system 49 has been included to aid in the discussion of prism-lens device 50.
Laser bar 10 is shown oriented so that laser bar axis 14 is parallel to the x-axis and laser beams 20 and 21 propagate in the z-direction. Note that when they are emitted from laser bar 10, laser beams 20 and 21 have their u-components aligned parallel to one another while their v-components are collinear. During operation of the optical system, laser beams 20 and 21 enter prism-lens device 50 through its front surface 52.
Laser beam 21 is incident upon a first reflecting surface 54 oriented at an angle to its direction of propagation. In the example provided, first reflecting surface 54 makes a 45.degree. angle with both the x-y plane and the y-z plane. This produces a reflected laser beam 21a which, in turn, is incident upon a second reflecting surface 56, oriented at an angle to the direction of propagation of laser beam 21a. In the example provided, second reflecting surface 56 makes a 45.degree. angle with both the y-z plane and the x-z plane. This produces a rotated laser beam 21b which passes into a lenslet 58. Lenslet 58 has a curvature only in the x-y plane and acts to collimate rotated beam 21b in the x-y plane.
In a similar sequence, laser beam 20 is incident upon a first reflecting surface 53 which makes a 45.degree. angle with both the x-y plane and the y-z plane. A reflected laser beam 20a is produced and, in turn, is incident upon a second reflecting surface 55 which makes a 45.degree. angle with both the y-z plane and the x-z plane. A rotated laser beam 20b is produced which passes into a lenslet 57. Rotated laser beam 20b emerges from lenslet 57 with its u-component collinear with the u-component of rotated laser beam 21b, and with its v-component aligned parallel to the v-component of rotated laser beam 21b. In this manner, prism-lens device 50 acts to rotate laser beams 20 and 21 by 90.degree. about their respective axes of propagation as a consequence of the two mirror reflections performed on the laser beams and collimates them in one azimuth.
In a second embodiment of the invention disclosed by Endriz, shown in FIGS. 5A and 5B, a first mirror 62 and a second mirror 64 are formed in a monolithic device 60. Monolithic device 60 is used for transforming the outputs of a multiple laser source in a manner similar to that of prism-lens device 50 described above. An incoming laser beam 20', confined within a laser cavity 66, is incident upon a first mirror 62. In the example provided, the direction of propagation of laser beam 20', indicated by the w-axis of a set of laser-beam uvw-coordinates 19', is coincident with the z-axis of an embodiment xyz-coordinate system 69. First mirror 62 is here shown at an angle of 45.degree. to both the x-y plane and the y-z plane. This produces a reflected laser beam 20a', that propagates in the x-direction to strike a second mirror 64. Second mirror 64 is here shown at an angle of 45.degree. to both the x-z plane and the y-z plane. This, in turn, produces a rotated laser beam 20b', propagating in the y-direction, which may then pass into a microlens 68 as shown in FIG. 5B. As can be seen, rotated laser beam 20b' has been rotated 90.degree. about its axis of propagation in a manner similar to the laser beam rotation performed by prism-lens 50 of FIG. 4.
The reference states that prism-lens device 50 can be assembled using precision alignment techniques, and that ion milling and another, more complex, technique are used to fabricate the mirrors in monolithic device 60. It can be appreciated that the embodiments presented above require complex fabrication methods and precise alignment owing to their multi-faceted nature.
Consequently, a need continues for a laser system in which the optical coupler used for reformatting the outputs of a multiple laser source is simpler to fabricate and utilize than prior art devices, and it is a primary object of the present invention to provide such a coupler.
It is another object of the present invention to provide an optical coupler which simply and efficiently combines the outputs of multiple laser sources into a single high-power laser beam.
It is a further object of the invention to provide such an optical coupler which can be formed without the necessity for complex fabrication processes.
It is a further object of the invention to provide such an optical coupler in which the output beam can be efficiently coupled into an optical fiber core.
It is yet another object of the invention to provide an easily-fabricated optical device for usefully modifying a plurality of light beams.
Other objects of the invention will, in part, appear hereinafter and will, in part, be apparent when the following detailed description is read in connection with the drawings.